Membrane carburetors and are known per se and they include a control membrane defining a partition wall between a fuel-filled fuel chamber and a compensating chamber. The compensating chamber is, as a rule, vented to the atmosphere. The combustion air flows in through the intake channel into the combustion chamber of the engine and generates an underpressure in the intake channel. This underpressure causes the fuel to enter the intake channel from the fuel chamber. The fuel passed into the intake channel leads to an underpressure in the fuel chamber whereby the control membrane is moved and a control valve is opened by means of which fuel is resupplied to the fuel chamber.
The combustion air drawn in by suction is drawn through an air filter for cleaning and the material of this filter becomes contaminated with increasing operational use whereby an underpressure builds up at the clean-air side of the air filter and this underpressure continuously increases with increasing contamination. The combustion air flowing through the air filter and via the intake channel into the combustion chamber therefore decreases in quantity and the underpressure in the intake channel increases thereby causing the fuel/air-mixture to become richer. A richer fuel/air-mixture leads to an incomplete combustion and thereby to a reduced quality of the exhaust gas with increased fuel consumption.